One type of flat panel display is known as a cold cathode field emission display (FED). A cold cathode field emission display uses electron emissions to illuminate a cathodoluminescent screen and generate a visual image. A single pixel 10 of a prior art field emission display is shown in FIG. 1A. The pixel 10 includes a substrate 11 having a conductive layer 12, and an array of emitter sites 13 on the conductive layer 12. Although each pixel 10 typically contains many emitter sites (e.g., 4-20 for a small display and several hundred for a large display), for simplicity only one emitter site 13 is shown in FIG. 1A. An extraction grid 15 is associated with the emitter sites 13 and functions as a gate electrode. The grid 15 is electrically isolated from the conductive layer 12 by an insulating layer 18. The grid 15-conductive layer 12-substrate 11 subassembly is sometimes referred to as a baseplate.
Cavities 23 are formed in the insulating layer 18 and grid 15 for the emitter sites 13. The grid 15 and emitter sites 13 are in electrical communication with a power source 20. The power source 20 is adapted to bias the grid 15 to a positive potential with respect to the emitter sites 13. When a sufficient voltage differential is established between the emitter sites 13 and the grid 15, a Fowler-Nordheim electron emission is initiated from the emitter sites 13. The voltage differential for initiating electron emission is typically on the order of 20 volts or more.
Electrons 17 emitted at the emitter sites 13 collect on a cathodoluminescent display screen 16. The display screen 16 is separated from the grid 15 by an arrangement of electrically insulating spacers 22. The display screen 16 includes an external glass face 14, a transparent electrode 19 and a phosphor coating 21. Electrons impinging on the phosphor coating 21 cause the release of photons 25 which forms the image. The display screen 16 is the anode in this system, and the emitter sites 13 are the cathode. The display screen 16 is biased by the power source 20 (or by a separate anode power source) to a positive potential with respect to the grid 15 and emitter sites 13. The potential at the display screen 16 is termed herein as an anodic potential. In some systems the potential at the display screen 16 is on the order of 1000 volts or more.
One problem that occurs during operation of a field emission display is known as "emission to grid". Emission to grid refers to an undesirable flow of electrons from the emitter sites 13 to the grid 15, or to other elements of the field emission display, such as the spacers 22. Emission to grid is particularly a problem during turn on (power on), and turn off (power off), of the field emission display.
Emission to grid during turn on is illustrated in FIG. 1B. During the turn on process, electrons 26 emitted from the emitter sites 13 can go directly to the grid 15 rather than to the display screen 16. This situation can lead to overheating of the grid 15. Emission to grid can also affect the voltage differential between the emitter sites 13 and the grid 15. In addition, desorped molecules and ions can be ejected from the grid 15 causing excessive wear of the emitter sites 13. Electron emission to grid 15 can also lead to electrical arcing 30 between the grid 15 and the conductive layer 12, or between the grid 15 and the emitter sites 13. In addition, electrons 26 emitted from the emitter sites 13 can strike the spacers 22 causing a charge build up on the spacers 22.
All of these problems decrease the lifetime, performance and reliability of a field emission display. Electron emission to grid is particularly a problem in consumer electronic products, such as camcorders, televisions and automotive displays, which are typically turned on and off many times throughout the useful lifetime of the product.
One reason for electron emission to grid, is that electron emission may have commenced from the emitter sites 13 before the large anodic voltage potential (V.sub.Anode) has been established at the display screen 16. Typically, the display screen 16 is a relatively large, relatively high voltage structure, that requires some period of time to reach full potential across its entire surface. In addition, the display screen 16 operates at a significantly higher voltage than any other component of the field emission display. Some period of time is required to ramp up to this operating voltage. Consequently, the display screen 16 can be at a low enough positive potential to allow electron emission to grid 15 to occur, as illustrated in FIG. 1B. Although this situation may only occur for a relatively short period of time, it can cause system problems as outlined above.
A related situation can also occur during turn on of the display screen 16 and grid 15 if the emitter sites 13 are not electrically controlled. If the emitter sites 13 are not limited during turn on, an uncontrolled amount of emission can occur causing the same problems as outlined above.
In addition, a similar situation exists during the turn off process for the FED cell 10 (i.e., power off). If power to the large positive potential at the display screen 16 is lost prior to termination of electron emission from the emitter sites 13, then electron emission to grid, as illustrated in FIG. 1B, can occur.
The present invention is directed to an improved field emission display and control circuit constructed to prevent electron emission to grid.